When a fossil fuel such as coal, petroleum, natural gas, etc., or waste is combusted, nitrogen oxides (NOx) are included in the exhaust gas, and such nitrogen oxides are a pollutant that becomes a main cause of air pollution, and thus necessarily should be removed before being discharged into the atmosphere.
For example, currently, selective catalytic reduction (SCR) is being widely used so as to remove nitrogen oxides included in exhaust gas. The selective catalytic reduction is a method of spraying ammonia or urea at the front end of a selective catalytic reduction (SCR) catalyst to induce the following chemical reaction, thus passing nitrogen oxides in exhaust gas through the catalyst together with ammonia and converting them into non-hazardous water and nitrogen.4NO+4NH3+O2→4N2+6H2O  (1)
In general, when coal or heavy oil is combusted in a boiler, sulfur dioxide (SO2) and sulfur trioxide (SO3) are generated, and among them, sulfur dioxide is partially oxidized into sulfur trioxide when passing through the selective catalytic reduction (SCR) catalyst as follows, and thus, when SCR denitrification facilities are installed, a total concentration of sulfur trioxide in the exhaust gas increases.
Since the increase in sulfur trioxide impairs facilities at the back end of the SCR catalyst, the conversion rate of SO2 into SO3 is limited to within 1%, and recently, when the concentration of sulfur (S) in fuel is high, the conversion rate into SO3 has been limited to within 0.7%.2SO2+O2→2SO3  (2)
Moisture exists in exhaust gas, and a part of ammonia introduced into the SCR catalyst reacts with sulfur trioxide and water as follows, thus forming ammonium sulfate.NH3+SO3+H2O→NH4HSO4, (NH4)2SO4  (3)
Ammonium sulfate is coated on the surface or gap, pores, etc. of a catalyst, to deteriorate catalytic activity. Since the catalyst pores function to increase the surface area where nitrogen oxide and ammonia can react, if the catalyst pores are blocked, catalyst performance may be deteriorated.
If the catalyst surface is thereby poisoned, the poisoned catalyst should be withdrawn from the reactor of denitrification facilities, and the poison of the catalyst should be removed to regenerate the catalyst by chemical treatment or the catalyst should be discarded.
The catalyst regeneration by chemical treatment has disadvantages in that expensive chemical solutions should be used, a high cost is required to treat generated waste water, and during the catalyst generation, moisture penetrates into the catalyst to deteriorate catalyst strength, and catalytically active materials such as V2O5, etc. are eluted. Further, in the case of the catalyst regeneration by chemical cleaning, the operation of a boiler should be necessarily stopped, thus generating enormous process losses.
In order to overcome the above-described problems, the present inventors have suggested a technology for solving catalyst poisoning by pollutants such as ammonium sulfate, etc. by installing a dry ice fine particle spray device at the front end of a catalyst inlet, in Korean Patent No. 10-1024845.
However, although catalyst regeneration is easily achieved with only the dry ice fine particle spray technology when only ammonium sulfate exists as a poison, when catalyst poisoning by particles such as uncombusted carbon, etc. increases due to the use of low quality fuel, a significant number of catalyst holes are blocked by uncombusted carbon powder and ammonium sulfate, and thus, when dry ice fine particles are sprayed, the fine particles flow back, thereby deteriorating the catalyst regeneration effect.
Also, when an alkali metal such as Na, K, etc. and P2O5, etc. are included in coal, heavy oil, or biofuel, it has been substantially technically difficult to completely remove catalyst poisons by only using a cleaning technology using dry ice fine particle spraying.
Thus, there is demand for the development of a catalyst regeneration method that can exhibit improved catalyst poison removal force, and that can exhibit denitrification performance equivalent to the initial catalyst.
(PATENT DOCUMENT 1) Korean Registered Patent No. 10-1024845 (Registration date: Mar. 18, 2011)